1. Field of the Invention
The present invention relates to ultrasonic imaging. More particularly, the present invention relates to creating sectional views from volumetric ultrasound data and superimposing positional data for an interventional device, such as a catheter.
2. Background of the Invention
Many medical procedures involve inserting a catheter, or other interventional device, into a patient for a variety of purposes. One such procedure is cardiac catheterization for an angiogram.
To observe the condition of blood vessels within the heart, radio-opaque dye is injected into the bloodstream through the catheter and an x-ray image is taken. Typically, this procedure involves inserting a catheter into an artery in the patient""s groin area and guiding the catheter through the arterial system and the aorta to the heart. In order to position the catheter and monitor its location within the patient, one or more x-ray images may be taken prior to injecting dye for the angiogram.
A catheter may be directly visible in an x-ray image, however, a heart typically is not. In order to determine the location of the catheter relative to the heart, dye may be injected through the catheter into the bloodstream. The dye acts as a contrasting agent allowing the heart to be located, by imaging the dye flowing through it. The image of the heart is then xe2x80x9cfrozenxe2x80x9d at a point in time when the dye location (and concentration), provides the best image. Often, this snapshot image of the heart is traced on a transparent overlay which is then superimposed on a display monitor. After the dye concentration decreases, the overlay may be used to track the catheter location and movements relative to the heart, as indicated by the overlay. There are two disadvantages to this process. First, the image of the heart, made visible with the dye, is only temporary, requiring the overlay to track the catheter movement. Second, if the patient""s position changes, the heart imaging must be repeated.
In addition to locating a catheter relative to a heart for an angiogram, there are many other medical procedures where an interventional device is inserted into a patient. In most, if not all, such procedures, locating and positioning the interventional device within a patient is extremely important.
Multiple views, or x-ray orientations, of a patient are often desirable to visualize objects and locations in three dimensions. Although there are many possible combinations of such views, some are favored by medical personnel for particular procedures. For cardiac imaging, the right anterior oblique (RAO) and the left anterior oblique (LAO) views are common. FIG. 1 shows a schematic sectional depiction of a prone patient 10 and x-ray sources 12 and 14. The RAO and LAO views are taken from the patient""s right and left, respectively, of the anterior (front) of the heart. The oblique angle, shown as angles 16 and 18, is the measure of how far off the vertical an x-ray source is oriented. The multiple views are often taken with a single x-ray source that is repositioned between positions 12 and 14. An imaging system that readily presents RAO and LAO views allows medical personnel to view images in a familiar format, as this is what is presently standard in the art. Multiple preferred views for other procedures might similarly be pre-defined.
There are problems with present practices. First, x-rays and fluoroscopy produce radiation, and the effect of this radiation over the long term may be harmful. This is especially true for the medical personnel subjected to repeated radiation exposures over the course of an entire career.
In addition, the radio-opaque dye may harm the patient. For example, an angiogram is often performed on patients with serious heart problems. Injecting chemicals that may cause sensitivity, or an allergic reaction, directly into the heart of such a patient may cause a serious problem, and there is the possibility of kidney damage from the dye.
Finally, x-ray techniques require complex x-ray equipment and the costly overhead associated with such equipment. While this may not be a primary concern in a well equipped modem hospital, it is of concern in less developed or remote locations.
Ultrasound based imaging methods offer some potential advantages over x-ray based imaging methods. Ultrasound methods do not expose the patient or medical personnel to radiation and may reduce or eliminate the need for costly x-ray equipment. Also, the ability of ultrasound methods to directly image soft tissue and blood vessels, without the need for radio-opaque dye as a contrasting agent, eliminates the potential dye-related harms to a patient.
It would be desirable to have an imaging system that could eliminate, or serve as a partial substitution for, x-ray and fluoroscopy methods of imaging in procedures such as angiography and tissue biopsies. Preferably, such a system would present medical personnel with multiple views that are displayed simultaneously.
U.S. Pat. No. 4,173,228, issued to Steenwyk and Childress on Nov. 6, 1979, describes a catheter locating device (xe2x80x9cthe ""228 patentxe2x80x9d). The ""228 patent uses an induction coil adjacent to the catheter tip, and a remote sensing device to monitor the amplitude and phase of signals induced in the coil, as a means of detecting the catheter location. However, an electrical signal from an induction coil is not well suited for detection by ultrasound imaging equipment. The ""228 patent does teach one technique for locating a catheter, but it lacks the ability to directly map that location to a 3-D ultrasonic image of tissues within the body. Instead, such a device determines a catheter location relative to a position on the skin surface and a depth estimate, based on the magnitude of the signal received at the skin surface. It would be desirable to locate the catheter relative to internal body tissues, not the skin.
U.S. Pat. No. 5,515,853, issued to Smith, et al. on May 14, 1996, describes a 3-D ultrasound tracking system based on triangulation (xe2x80x9cthe ""853 patentxe2x80x9d). Using a network of at least four piezoelectric transducers exchanging signals, and an integrated personal computer (PC) as a digital controller, the ""853 patent accurately measures the relative distances between the transducers. However, like the ""228 patent discussed above, an apparatus based on the ""853 patent lacks the ability to map the location of a catheter to a 3-D ultrasonic image of tissue within the body. Instead, the reference frame for catheter location information is the location of other piezoelectric transducers. For example, a series of transducers contained in a chest harness around a patient may be used to triangulate the position and orientation of a catheter relative to the chest harness, by measuring the distances to transducers mounted in the catheter. The ""853 patent does briefly mention the possibility of xe2x80x9coverlayingxe2x80x9d transducer location information on a video loop, created from x-rays or ultrasound, to facilitate visualization of the catheter location. However, there is no teaching of how this overlay process might be performed. In contrast, an embodiment of the present invention is directed to describing the catheter location relative to tissue within the body in one or more 2-D views.
U.S. Pat. Nos. 5,817,022 and 5,868,673, (xe2x80x9cthe ""022 and ""673 patentsxe2x80x9d) issued to Vesely on Oct. 6, 1998, and on Feb. 9, 1999, respectively, are both, continuations-in-part from the application that became the ""853 patent. Both also claim 3-D ultrasound tracking of interventional medical instruments by triangulating between a transducer attached to the medical instrument and a network of transducers either inside a patient""s body or on the body surface. It would be desirable to simplify the apparatus so that a network of transducers is not required.
The ""022 patent locates a medical instrument relative to the network of transducers in 3-D, and then displays a 2-D ultrasound image within the 3-D coordinate system. The ""022 patent describes an xe2x80x9cimaging modality systemxe2x80x9d, that may acquire 2-D, 3-D or 4-D image data, an xe2x80x9cimage registration systemxe2x80x9d to register the position the [medical] instrument within the image data and a xe2x80x9cuser interfacexe2x80x9d to perform a desired function, such as selecting a particular view for display. However, there are no detailed descriptions of these elements, or teachings which would permit one of ordinary skill in the art to create a system combining such elements.
The ""673 patent describes a system for performing surgery while tracking the location of a medical device relative to a network of transducers, on or within the body. The medical device location is then mapped to the network of transceivers, but not directly to any portion of the body tissue.
Ultrasound image data for an object, such as a heart, consists of the xe2x80x9cechodensityxe2x80x9d sampled at multiple locations within an object. This echodensity can be thought of as somewhat equivalent to the color or intensity of a visual image. The echodensity is a function of the sound velocity within the tissue, as well as the tissue density. Although a 3-D image of echodensity, with continuous gradations along three dimensions, may lack the clarity of the sharp edges in a solid model, the gradations contain a significant amount of information about the tissue. A solid modeling approach to presenting a 3-D ultrasound image typically uses a threshold value. That is, all echodensity values are divided into two bins, those above and those below the threshold. Values above the threshold are opaque and those below are transparent. This threshold technique adds clarity to the wide variations of the xe2x80x9cmurkyxe2x80x9d echodensity data, by distinguishing solid features within the data, at the expense of the detail stored in those many values.
TomTec Imaging Systems, formerly of Boulder, Colo., now TomTec Imaging Systems GmbH of Munich, Germany, produced a video tape describing a 3-D ultrasound imaging system (xe2x80x9cthe TomTec systemxe2x80x9d). TomTec Imaging systems, Clinical Three-Dimensional Echocardiography, VHS/NTSC, 19:20, 040-012.0, 07-96 (video tape). The TomTec system creates a 3-D cardiac image, with the use of an ultrasound transducer in the form of an endoscope, and produces an image of the surfaces of the heart, essentially a solid model. That is, the surface of the heart is displayed without any information about the internal muscle tissue. The TomTec system does not appear to have been commercially successful.
The ultrasound imaging system of the present invention superimposes sectional views created from volumetric ultrasound data and the location data for an intervention device, such as, but not limited to, a catheter. The position of catheter may be shown, in one or more views, relative to organs and tissues within a body as the catheter is moved. The catheter positional data is updated continuously and is superimposed on tissue images that may be updated less frequently, resulting in real-time or near real-time images of the catheter relative to the tissues.
The superimposed images allow medical personnel to perform procedures such as angiograms with minimal exposure of patients to x-rays and contrasting dye. However, the look and feel of the familiar fluoroscopy-like imaging may be maintained.